This application outlines a 5 year plan to train Dr. William Swiggard for an academic career studying the pathogenesis of HIV infection. Dr. Swiggard has completed residency training in Internal Medicine and a Clinical Fellowship in Infectious Diseases. He is now working in the laboratory of Dr. James Hoxie, one of his sponsors. His research interests center upon events that immediately follow HIV-1 exposure, during which dendritic cells (DCs), the primary antigen-presenting cells responsible for the activation of resting T cells, bind the virus and subsequently transfer it to T cells by a process called "trans-infection". Dr. Swiggard has a strong background in cellular immunology, having received a Ph.D. in this field under Dr. Ralph M. Steinman in the Department of Cellular Physiology and Immunology at The Rockefeller University. While this experience poises him to investigate immune responses by primary leukocytes, he lacks the experience in virology and HIV molecular biology necessary to efficiently attack questions of viral pathogenesis independently. A comprehensive research and training program is being established for him within the Division of Infectious Diseases. His co-sponsors are Dr. James A. Hoxie and Dr. Michael H. Malim, both recognized authorities on molecular HIV-1 pathogenesis. In addition, an advisory board of prominent scientists will insure his development. Immature DCs circulate in blood, and populate all of the epithelia that are traversed by HIV-1 during sexual exposures. Although they have been implicated in critical events early in HIV-1 infection, and express all of the receptors necessary for HIV-1 binding and fusion, endogenous circulating DCs appear to resist productive HIV-1 infection. A novel lectin expressed solely by DCs, DC-SIGN, has been implicated in both viral acquisition and "trans-infection", but has not been studied directly in endogenous circulating DCs that have not been cultured and treated with cytokines. Dr. Swiggard proposes to study HIV-1 interactions with endogenous blood DCs, via 3 specific aims: (1) use of blocking antibodies to DC- SIGN followed by quantitative PCR to probe the role of this binding protein in HIV-1 acquisition and "trans-infection" by DCs; (2) deposition of large numbers of HIV-1 virions on DCs using centrifugal inoculation, followed by quantitative kinetic PCR assays that permit the study of rare post-entry events; (3) use of in situ hybridization techniques and kinetic PCR to study viral trafficking in DCs. These studies will provide important new insights into the HIV disease process, and will concurrently provide Dr. Swiggard with the expertise necessary for him to become an independent HIV investigator.